Concern about Sir Menzies Campbell's impact as Liberal Democrat leader will be strengthened today by Guardian/ICM poll findings showing a sharp drop in support for his party to a four-year low.

As MPs start their two-month break from the Commons, the Lib Dems emerge as the main losers in a poll which also sees the Conservatives on a 13-year high, equalling their best Guardian/ICM rating since Black Wednesday in 1992.

Only 17% of voters said they would back the Lib Dems in an immediate general election, down four points on a month, and six points below the party's rating a year ago when it was led by Charles Kennedy. The Lib Dems have not scored 17% or below in a Guardian/ICM poll since March 2002, before the Iraq war began to cause some Labour voters to switch to the Liberal Democrats.

Mr Kennedy was persuaded to stand down at the start of this year by Lib Dem MPs concerned at his lack of impact despite Labour's difficulties. But this month's poll puts Labour up three points, to 35%, narrowing the gap on the Conservatives, who climb two points to 39%.

That is close to the 40% rating which Conservatives believe could allow them to challenge for power in a general election. The Conservatives last scored 39% in a Guardian/ICM poll in January 1993, as they recovered briefly before a decade-long collapse in public support.

Lib Dems can point to the recent Bromley and Chislehurst byelection, in which the party nearly beat the Conservatives, as a sign of their underlying strength despite this month's findings. The party also won the Fife byelection in February and ran Labour neck and neck for second place in the May local elections. Since last year's general election - in which the party won 22.6% of votes - Lib Dems have averaged 21% in Guardian/ICM polls.

Labour will be heartened to enter the summer recess climbing in the polls, despite the shocks of recent months, including the arrest of Lord Levy and new pressure on John Prescott. The party's 35% rating suggests that Labour can rely on the support of around a third of the electorate, despite sustained bad news.

Meanwhile voters appear to welcome Labour's promise of a debate and parliamentary vote on the replacement of Britain's Trident nuclear deterrent, expected to come later this year. Only a small majority, 51%, want to replace it, with 39% opposing renewal.

· ICM interviewed a random sample of 1,001 adults over 18 by telephone on July 21-23. Interviews were conducted across the country and the results have been weighted to the profile of all adults. More data at icmresearch.co.uk

How the parties fared - and the pitfalls that await them

Labour:

When Tony Blair staged only a modest reshuffle on May 6 2005, his 52nd birthday, he was reeling from an election which had cut his Commons majority from 165 to 64. It was a warning of the uphill struggle ahead for his own and Labour's survival. Two cabinet grandees have since been forced out.

Since the election No 10 has lost four whipped votes and been forced to modify key bills. The prime minister has seen the Home Office engulfed in crises and been embroiled himself in controversy over cash-for-peerages about which police will interview him shortly.

Beset by his own problems, John Prescott has survived as Mr Blair's lightning conductor. Can they survive to retire together next summer, the likely game plan? Faced with media and Labour demands for Mr Blair's early departure, even Gordon Brown's unchallenged succession is in doubt.

The chancellor has made speeches, defining himself mostly in modernising terms, without revealing the post-Blair initiatives he is said to be planning. Alan Johnson emerged as frontrunner for Mr Prescott's job.

But Mr Blair is still standing. Nearly 10 years in power, yet the Tory poll lead is fragile, say loyalists.

Challenges ahead

The Manchester party conference where Blair will face renewed pressure to quit; cash-for-peerages police inquiry and other lurking scandals; external economic threats to undermine Mr Brown's record just as he prepares to move to No 10; policy battles like NHS reform and nuclear power stations; the Middle East.

Conservatives:

After Michael Howard bequeathed his three-times defeated party with a leadership contest in May 2005 no one predicted that a year later David Cameron would dominate his party and the polls; detoxifying the Tories' lethal image without moving its policies "one inch", as Tim Bell, Margaret Thatcher's image guru, said admiringly yesterday. His rise has been the political event of the year.

The Tory leader's discarded tie became a symbol of affable Etonian style, as the 39-year-old father of three voiced concerns about poverty, the ozone hole and better public services. "Hug a hoodie" was a Labour jibe, but Mr Cameron's presentational skills (he also hugged huskies) got him attention.

Absent from Mr Cameron's shop window has been policy. His few attempts - child care tax relief and a British human rights act - have been hammered. But aides ask: "What's the hurry"? They dismiss Tony Blair as the past and chip away at Gordon Brown as a "roadblock to reform".

They must also complete their grassroots consultation on revised "aims and values" for a winter ballot, before next year's tricky policy overhaul. Douglas Alexander, minister for Europe, yesterday called it the old rightwing song repackaged - or "lipstick on a pig".

Cameron's conundrums

This autumn's roadshows, which must end in a membership ballot to endorse the leadership's approach; rightwing restlessness at the Bournemouth conference in October; policy shifts get rubbished, not least by other Tories; Gordon Brown takes over and sweeps all before him

Liberal Democrats:

Under Charles Kennedy they emerged with a 70-year record, 62 Commons seats, but drifted until the leader's drinking was used to force him out. A scandal-soaked leadership contest saw Sir Menzies Campbell, now 65, easily beat the two surviving rivals without damaging poll ratings, only to start drifting himself as Mr Cameron grabbed all the publicity.

With the Tory revival nibbling at support and Sir Menzies failing to shine in the Commons, the party tried to interest voters in "fairer, not higher" plans for taxing the rich and heavy carbon footprint types. Too complicated for MPs, let alone voters? Some wondered.

Simon Hughes, a defeated leadership candidate, gave his leader until the party conference to sort himself out, amid rumblings that Sir Menzies might have to stand aside before the next election if he did not.

Liberal Democrats took comfort from one byelection gain in Gordon Brown's backyard of Fife, one near-miss in Bromley, and their rivals' similar misery. Even Labour and the Tories admit the y do better in real elections and lose ground during a crisis like Lebanon.

Problems they may face

Sir Menzies Campbell's conference speech in Brighton bombs as badly as TV and Commons appearances, triggering panic over falling poll shares; another fight over the role of the market, despite attempts to water down the policy booklet, Orange Book 2; Kennedy ends his post-leadership silence to make clear he's not finished yet; Cameron's success recaptures more disillusioned voters; peace unexpectedly comes to Iraq and wider Middle East, undermining Lib Dems' unique selling point with angry Labour voters in 2005